Transcript Lecture22_deformation

I. Deformation
Shaping the Earth’s
Crust
Already talked about
plate tectonics
Large-scale movement
of Earth’s lithosphere
Structural geology:
study of crustal
deformation &
mountain building
I. Force, Stress, and Strain
A.
Stress: the force applied
per unit area
Stress = σ = F/A
(Force = Mass x Acceleration)
Use example of an axe, sharp
side vs dull side and hockey
B. Strain: the distortion or
deformation.
The change in size or shape of a
rock
The deformation that occurs due
to stress
II. Types of Differential Stress
Stress applied unequally in different directions
1. Compressional
2. Tensional
3. Shear
II. Types of Differential
Stress
Stress applied unequally
in different directions
A. Compression: stress
that shortens a rock
body, pushes it together
Associated with plate
collisions, convergence
Causes rocks to crumple,
thicken vertically and
shorten laterally
II. Types of Differential
Stress
Stress applied unequally
in different directions
B. Tension: stress that
stretches or extends
rocks
Associated with divergent
plate boundaries
Causes rocks to thin
vertically and lengthen
laterally
II. Types of Differential
Stress
Stress applied unequally in
different directions
C. Shear: stress that pushes
rocks past each other in
parallel but opposite
directions
Associated with transform plate
margins
Slices rocks into parallel blocks,
breaks and displaces preexisting
rocks & structures
(deck of cards example)
III. Types of Deformation
When a rock is subjected
to stress, they can strain
in different ways
A. Elastic deformation =
recoverable
When minor stress is applied,
a rock may strain slightly,
but then return to its original
shape after the stress is
removed
Rubber band strains, but
returns to its original shape,
rocks usually won’t take as
much stress as a rubber
band
III. Types of Deformation
B. Brittle deformation
Rocks crack or rupture
Faults
The actual atomic bonds
within the rock are broken
along the zone of max
stress = where the crack
appears
Occurs under conditions of
low temp and pressure
Also occurs when stress is
applied quickly, a quick
swing of a hammer
III. Types of Deformation
C. Plastic (ductile)
deformation
Folds
An irreversible change in shape or
volume that occurs without the
rock breaking
Atoms rearrange themselves on
a microscopic scale, from areas of
maximum stress to areas of lower
stress (i.e. toothpaste oozes away
from pressure)
Occurs under conditions of higher
temp and pressure
Draw folds caused by
horizontal compression
Sometimes a rock begins
experiences ductile deformation
but if the stress increases it may
fail by brittle deformation
III. Types of Deformation
C. Plastic (ductile) deformation
Folds
An irreversible change in
shape or volume that occurs
without the rock breaking
Plastic (ductile): low rates of
strain, exceed yield point
Silly Putty Example: Elastic,
roll into ball and bounce
Brittle: high rates of strain,
rocks break or fracture,
exceed yield point
Stephen Marshak
IV. Factors affecting rock
deformation
A. Heat (blacksmith heats
metal to work with it, so its
ductile)
1. low temperature  brittle
2. high temperature 
ductile/plastic
B. Pressure
1. low pressure  brittle
2. high pressure  ductile
IV. Factors affecting rock
deformation
C. Time dependent (just as
with silly putty)
1. less time  brittle
2. more time (same stress) 
ductile
D. Rock Type or composition
1. Some mineral bonds are
simply stronger than others
(all other factors being
equal)
Salt is weak  ductile
basalt is strong  brittle
2. The presence of water helps
rocks be more ductile